A number of bacteria, and particularly strains of Acetobacter, can be cultivated to produce bacterial cellulose. In the presence of sugar and oxygen, cells of Acetobacter synthesize cellulose extracellularly in the form of fibrils attached to the cell. The fibrils produced by cells incubated in a static culture intertwine with one another to form a hydrophilic network known as a pellicle. See, U.K. Patent Publication No. 2,131,701. This pellicle forms on the air/liquid interface of the motionless and undisturbed culture which is usually contained in shallow trays. Coherent gel-like microbial cellulose pellicles have many uses such as in wound dressings or paper, after removal of the cells.
Bacterial cellulose can also be produced in traditional stirred or agitated bioreactors. However, a non-pellicular form of bacterial cellulose is typically produced by this type of culturing. This production method is highly susceptible to strain instability which is demonstrated by the cell's loss of ability to produce cellulose and gradual cell overgrowth. Valla et al., Cellulose negative mutants of Acetobacter xylinum, Journal of General Microbiology, 128, 1401 (1982). Nevertheless, sustained production of reticulated bacterial cellulose under agitated conditions for over 70 hours with mutagenized and selected strains has been reported by Johnson, U.S. Pat. No. 4,863,565.
Byron, U.S. Pat. No. 5,273,891, disclose a four-step process for producing microbial cellulose in a stirred batch culture. Airlift fermentors, which use air bubbles rather than impellers for mixing, have also been adapted for bacterial cellulose production by Okiyama et al., Bacterial cellulose I: Two-Stage Fermentation Process for Cellulose Production by Acetobacter aceti, Food Hydrocolloids, 6, No. 5, 471-477. (1992)).
Film bioreactors have been used in wastewater treatment in which growing cells are attached to a structural element of the reactor to form a film. Cell growth is promoted either by flowing nutrients across the film or by moving the film contained in the structural element into a nutrient filled vessel. Two types of film bioreactors are the trickling filter where the film is stationary and media percolates through the film and the rotating biological contactor (RBC) where the film is cultured on a rotary disk which rotates through the medium.
A third type of film bioreactor has also been used in the production of filamentous fungi. See, Heiland, U.S. Pat. No. 5,246,854. This attached growth biological reactor uses a rotating cylinder to which the filamentous fungi attach while the cylinder is partially submerged in a trough filled with nutrient media.
However, non-static film bioreactors have not been used in the production of microbial cellulose. The use of the film bioreactor, as explained in the present invention, provides highly hydrated pellicular and film forms of microbial cellulose which can be processed in specialized manner, during or after growth because of the processes described herein.